diff --git a/include/nlohmann/json_fwd.hpp b/include/nlohmann/json_fwd.hpp
index 32bde590f8..f9ff44369b 100644
--- a/include/nlohmann/json_fwd.hpp
+++ b/include/nlohmann/json_fwd.hpp
@@ -10,9 +10,11 @@
 #define INCLUDE_NLOHMANN_JSON_FWD_HPP_
 
 #include <cstdint> // int64_t, uint64_t
+#include <functional> // less
 #include <map> // map
 #include <memory> // allocator
 #include <string> // string
+#include <utility> // pair
 #include <vector> // vector
 
 #include <nlohmann/detail/abi_macros.hpp>
@@ -63,13 +65,14 @@ using json = basic_json<>;
 
 /// @brief a minimal map-like container that preserves insertion order
 /// @sa https://json.nlohmann.me/api/ordered_map/
-template<class Key, class T, class IgnoredLess, class Allocator>
-struct ordered_map;
+template<class Key, class T, class IgnoredLess = std::less<Key>,
+         class Allocator = std::allocator<std::pair<const Key, T>>>
+                 struct ordered_map;
 
-/// @brief specialization that maintains the insertion order of object keys
-/// @sa https://json.nlohmann.me/api/ordered_json/
-using ordered_json = basic_json<nlohmann::ordered_map>;
+         /// @brief specialization that maintains the insertion order of object keys
+         /// @sa https://json.nlohmann.me/api/ordered_json/
+         using ordered_json = basic_json<nlohmann::ordered_map>;
 
-NLOHMANN_JSON_NAMESPACE_END
+         NLOHMANN_JSON_NAMESPACE_END
 
 #endif  // INCLUDE_NLOHMANN_JSON_FWD_HPP_
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index 09fffb3bd1..83b1d93f71 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -3403,9 +3403,11 @@ NLOHMANN_JSON_NAMESPACE_END
     #define INCLUDE_NLOHMANN_JSON_FWD_HPP_
 
     #include <cstdint> // int64_t, uint64_t
+    #include <functional> // less
     #include <map> // map
     #include <memory> // allocator
     #include <string> // string
+    #include <utility> // pair
     #include <vector> // vector
 
     // #include <nlohmann/detail/abi_macros.hpp>
@@ -3457,7 +3459,8 @@ NLOHMANN_JSON_NAMESPACE_END
 
     /// @brief a minimal map-like container that preserves insertion order
     /// @sa https://json.nlohmann.me/api/ordered_map/
-    template<class Key, class T, class IgnoredLess, class Allocator>
+    template<class Key, class T, class IgnoredLess = std::less<Key>,
+    class Allocator = std::allocator<std::pair<const Key, T>>>
     struct ordered_map;
 
     /// @brief specialization that maintains the insertion order of object keys
diff --git a/single_include/nlohmann/json_fwd.hpp b/single_include/nlohmann/json_fwd.hpp
index 29a6036d79..61578a3024 100644
--- a/single_include/nlohmann/json_fwd.hpp
+++ b/single_include/nlohmann/json_fwd.hpp
@@ -10,9 +10,11 @@
 #define INCLUDE_NLOHMANN_JSON_FWD_HPP_
 
 #include <cstdint> // int64_t, uint64_t
+#include <functional> // less
 #include <map> // map
 #include <memory> // allocator
 #include <string> // string
+#include <utility> // pair
 #include <vector> // vector
 
 // #include <nlohmann/detail/abi_macros.hpp>
@@ -164,13 +166,14 @@ using json = basic_json<>;
 
 /// @brief a minimal map-like container that preserves insertion order
 /// @sa https://json.nlohmann.me/api/ordered_map/
-template<class Key, class T, class IgnoredLess, class Allocator>
-struct ordered_map;
+template<class Key, class T, class IgnoredLess = std::less<Key>,
+         class Allocator = std::allocator<std::pair<const Key, T>>>
+                 struct ordered_map;
 
-/// @brief specialization that maintains the insertion order of object keys
-/// @sa https://json.nlohmann.me/api/ordered_json/
-using ordered_json = basic_json<nlohmann::ordered_map>;
+         /// @brief specialization that maintains the insertion order of object keys
+         /// @sa https://json.nlohmann.me/api/ordered_json/
+         using ordered_json = basic_json<nlohmann::ordered_map>;
 
-NLOHMANN_JSON_NAMESPACE_END
+         NLOHMANN_JSON_NAMESPACE_END
 
 #endif  // INCLUDE_NLOHMANN_JSON_FWD_HPP_